text: Redox gradients form based on resource availability and physiochemical conditions (pH, salinity, temperature) and support stratified communities of microbes. Microbes carry out differing respiration processes  (methanogenesis, sulfate reduction, etc.) based on the conditions around them and further amplify redox gradients present in the environment. However, distribution of microorganisms cannot solely be determined from thermodynamics (redox ladder), but is also influenced by ecological and physiological factors.
  Redox gradients form along contaminant plumes, in both aquatic and terrestrial settings, as a function of the contaminant concentration and the impacts it has on relevant chemical processes and microbial communities. The highest rates of organic pollutant degradation along a redox gradient are found at the oxic-anoxic interface. In groundwater, this oxic-anoxic environment is referred to as the capillary fringe, where the water table meets soil and fills empty pores. Because this transition zone is both oxic and anoxic, electron acceptors and donors are in high abundance and there is a high level of microbial activity, leading to the highest rates of contaminant biodegradation.
  Benthic sediments are heterogeneous in nature and subsequently exhibit redox gradients. Due to this heterogeneity, gradients of reducing and oxidizing chemical species do not always overlap enough to support electron transport needs of niche microbial communities. Cable bacteria have been characterized as sulfide-oxidizing bacteria that assist in connecting these areas of undersupplied and excess electrons to complete the electron transport for otherwise unavailable redox reactions.
  Biofilms, found in tidal flats, glaciers, hydrothermal vents, and at the bottoms of aquatic environments, also exhibit redox gradients. The community of microbes—often metal- or sulfate-reducing bacteria—produces redox gradients on the micrometer scale as a function of spatial physiochemical variability.
  See sulfate-methane transition zone for coverage of microbial processes in SMTZs.